A large number of methods and systems exist intended for faithful reproduction of the sound experienced by a listener at the recording position. The only one of these that is able to truly reproduce the stereo effect, i.e. the impression of the different sound sources originating from different spatial positions, is using stereo headphones. Listening to a recorded stereo sound using headphones, the listener may perceive a stereo image identical to the image that would have been perceived at the recording site. This method is however not suitable for reproduction of stereo sound to an audience consisting of more than one listener. To overcome this drawback, audio stereo reproduction systems comprising two, or more, loudspeakers are used for reproducing stereo sound to an audience. Most of these systems are based on a pair of widely spaced loudspeakers, and true reproduction of the stereo effect, both in terms of relative intensity between the sound perceived by the listerners' two ears and the time difference between these, can be perceived only at a single position in relation to the loudspeakers. This implies that only one listener in an audience can experience a truly correct stereo effect. All other members of the audience will therefore experience a distorted stereo effect. Different ways to widen the area over which the perceived stereo impression is nearly correct have been attempted, with varying degrees of success.
Physically separating the two loudspeakers a distance large enough for enabling reproduction of the stereo impression to at least one listener is generally impractical, and in certain cases impossible. Examples of such cases are single unit stereo radio or CD players with integral loudspeakers, or reproduction of stereo sound to several listeners in cars or small rooms. Adjusting the relative intensities of the side and the mid signals reproduced by an AB stereo system, to increase the perceived stereo width might improve the impression of stereo width, but might also distort the stereo image, and it is not recommended to shift the ratio between the two signals by more that 3 dB. Other methods of improving the perceived stereo effect from narrowly separated loudspeakers have also been suggested, but have proven to give limited effect.